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Probiotics, live microbial supplements marketed on the basis of improving the microbial “balance” in the human gut, account for a growing market–perhaps 10 billion Euro in Europe alone^(source).

While probiotics are apparently a market (and marketing success)–indeed it’s getting ever more difficult to buy yogurt that isn’t probiotic–whether there is even such a thing as an ideal microbial “balance” is not clear. What is “normal” in terms of gut flora is not well defined, and presumably it would differ between healthy individuals anyway.

Nonetheless, there is growing evidence of health benefits associated with taking probiotics, for example after taking antibiotics or in the reduction of diarrheal symptoms. The problem with marketing probiotics as health supplements is that the gastrointestinal conditions they are meant to treat don’t, for the most part, have validated biomarkers, so it’s difficult to conceive of appropriate trials^(ref).

A second problem is that even if they do work (which does seem increasingly likely), for whatever value of “work” we think is appropriate, we’re still not entirely sure how. There have been a few papers on probiotic mechanism in F1000 recently, including one I wrote up for The Scientist earlier in the year, and a recent piece on de-stressing mice^evaluation) . In fact, a search for probiotic mechanism turns up 36 evaluated articles at the time of writing.

The most recent of these is an hypothesis in BioEssays by Mark Lyte of Texas Tech, Probiotics function mechanistically as delivery vehicles for neuroactive compounds: Microbial endocrinology in the design and use of probiotics. Lyte proposes that probiotic bacteria manufacture and deliver neuroactive compounds such as acetylcholine, epinephrine and dopamine, which could, at least in part, account for how changing the gut flora can have beneficial effects on the brain.

Obviously, if this hypothesis is right (even partially), it would have very interesting implications for selection and design of probiotics in the clinical setting, let alone what you have with your cereal for breakfast.